1. Field of the Invention
The present invention generally relates to a method for preparing the high energy fuels, and more particularly to a method for preparing the low carbon number petrochemical products along with the high energy fuels from pyrolysis gasoline (pygas) used as feedstock.
2. The Prior Arts
The major constituent of the high-density liquid hydrocarbon fuels for propelling missile systems is Jet Propellant 10 (JP-10), which is a C10 cyclic hydrocarbon, and contains >98.5 wt % of exo-tetrahydrodicyclopentadiene (exo-THDCPD), and <1.5 wt % of endo-tetrahydrodicyclopentadiene (endo-THDCPD). JP-10 has the characteristics of high volumetric heating value, low freezing point high thermal stability, and low viscosity, and thereby no other aviation fuels having a similar boiling point as JP-10 can compete with JP-10 in fueling. Up to now, the high density fuel of JP-10 is chemically synthesized using the pure compounds as starting material, and which will cause high manufacturing cost. The price of JP-10 is sixteen to eighteen times of the prices of other aviation fuels. Therefore, JP-10 is almost exclusively used in missile applications.
In the prior art, JP-10 is prepared by first completely hydrogenating extra-high purity cyclopentadiene (CPD) dimer, namely dicyclopentadiene (DCPD, freezing point of 33.6° C.), as a precursor to yield the solid endo-isomer of the hydrogenated derivative, namely endo-THDCPD (freezing point of 77° C.), wherein the hydrogenation of DCPD is carried out in two stages (the first stage and the second stage). In the first stage, the dihydro derivative, namely dihydrodicyclopentadiene (DHDCPD, freezing point of 51° C.) was obtained by selective hydrogenation of DCPD in the presence of nickel or palladium catalyst at the temperature of 50-120° C. In the second stage, the solid endo-THDCPD (freezing point of 77° C.) was obtained by hydrogenation of DHDCPD in the presence of metal catalyst at the temperature of 130-220° C. Finally, the solid endo-THDCPD is isomerized to the liquid exo-THDCPD, namely JP-10 (freezing point <−79° C.), in an amount of more than 98.5% of the total product weight in the presence of an acidic catalyst. JP-10 is very expensive because it is obtained by chemical synthesis from extra-high purity DCPD using a batch reactor (referring to U.S. Pat. Nos. 3,381,046, 4,086,284, 4,107,223, and 4,270,014).
The amount of DCPD precursor (less than 1 to 2 million pounds per year) that is used in preparing JP-10 accounts for around 1% of the total global production of DCPD. DCPD is mainly used as monomers in the production of many copolymers. However, DCPD and CPD monomer thereof are mainly obtained from pygas that is a by-product of ethylene production from steam crackers. DCPD is produced by first separating the C5 fractions (the term C5 refers to the hydrocarbons containing five carbon atoms) from the pygas fed to a depentanizer, and then by dimerizing CPD present in the C5 fractions by heat soaking at the temperature ranging from 100 to 120° C. After distillation of the crude C10 fractions, DCPD is withdrawn from the bottom of the depentanizer, which can be separated from the undimerized C5 hydrocarbons withdrawn from the top of the depentanizer. The high-purity (>98-99%) DCPD usually is obtained by thermal cracking (pyrolysis) of 75-90% purity of crude DCPD to CPD at the temperature ranging from 170 to 172° C. and then distilling the CPD off from the hydrocarbon mixture. The thus obtained CPD distillate is dimerized during heat soaking at the temperature ranging from 100 to 120° C. to form DCPD. The production cost of DCPD is very high because DCPD is obtained by repeating the steps of thermal crack and dimerization.
The high-energy fuels of RJ-4 and RJ-4I (C12 cyclic hydrocarbons), which have the similar structures as that of JP-10, were also produced by chemical synthesis (referring to U.S. Pat. No. 4,398,978). RJ-4 and RJ-4I both comprise endo-tetrahydrodimethyldicylopentadiene (endo-THDMCPD) and exo-tetrahydrodimethyldicyclopentadiene (exo-THDMCPD) isomers, but in different ratio. RJ-4 and RJ-4I were obtained by using methylcyclopentadiene (MCPD, C6 cyclic hydrocarbons) as a precursor and following the same reaction mechanism as that for converting CPD to DCPD. RJ-4 or RJ-4I is prepared by first dimerizing MCPD to form dimethyldicyclopentadienes (DMCPD), followed by completely hydrogenating DMCPD to produce the endo stereo isomeric form of the tetrahydro derivative, namely tetrahydrodimethyldicyclopentadienes, wherein the complete hydrogenation of DMCPD is carried out in two stages. In the first stage, DMCPD is converted to endo-dihydrodimethylcyclopentadienes (endo-DHDMCPD), and in the second stage, endo-DHDMCPD is converted to endo-THDMCPD. Finally, portions of the endo-THDMCPD isomers are isomerized to the exo-THDMCPD isomers in the presence of acidic catalyst RJ-4I has a lower freezing point and viscosity than RJ-4 since RJ-4I has more exo-THDMCPD isomers produced than RJ-4 has (referring to U.S. Pat. No. 4,398,978).
According to the same reaction mechanism, as mentioned above, endo-tetrahydro CPD/MCPD is prepared by first codimerizing CPD (C5 cyclic hydrocarbons) with MCPD (C6 cyclic hydrocarbons) by heat soaking at the temperature ranging from 100 to 120° C. to form endo-CPD/MCPD codimer (C11 cyclic hydrocarbons), followed by completely hydrogenating endo-CPD/MCPD codimer to produce the endo stereo isomeric form of the tetrahydro derivative, namely endo-tetrahydro CPD/MCPD codimer (endo-THMDCPD). The complete hydrogenation of endo-CPD/MCPD codimer is carried out in two stages. In the first stage, endo-CPD/MCPD codimer is converted to endo-dihydro CPD/MCPD codimer (DHMDCPD), and in the second stage, endo-dihydro CPD/MCPD codimer is converted to endo-tetrahydro CPD/MCPD codimer (endo-THMDCPD). Finally, portions of the endo-THMDCPD isomers are isomerized to the exo-THMDCPD isomers in the presence of acidic catalyst. The compositions of the C11 high energy fuels and the method for preparing the same have been disclosed by U.S. Pat. No. 4,398,978. The C11 high energy fuels have combustion characteristics fallen between those of JP10 and RJ-4I. However, no specific fuel names were assigned to the C11 high energy fuels since the C11 high energy fuels which can be used as liquid missile fuels have not been manufactured in mass production by chemical synthesis so far.
On the other hand, the principal product from naphtha steam cracker is ethylene. However, other heavier by-products as petrochemical feedstocks are also produced, such as C4-C5 diolefins, styrene, methylstyrene, and DCPD in addition to benzene, toluene, and xylene. The pygas by-products from naphtha steam cracker are usually blended with gasoline or fuel if the naphtha steam crackers are small-sized ones without the need of petrochemical feedstocks. The composition of pygas blended with gasoline and fuel is different from the composition of pygas used as a petrochemical feedstock because the pygas, which is to be blended with gasoline or fuel, need to be further treated. These small-sized naphtha steam crackers usually have the two-stage hydrogenation equipments used for converting the C8− active constitutes (the term C8− refers to the hydrocarbons containing eight or less carbon atoms), such as diolefins and styrene, contained in the pygas into more stable products. For example, styrene is hydrogenated to ethylbenzene. Therefore, the hydrotreated pygas can be stabilized for later blending and storage, and moreover the coking and clogging will not occur in distillation equipments and oil pipelines during the follow-on treatments. During the second-stage hydrogenation, the residual olefins were completely saturated with hydrogen, and also the sulfur containing species present in the petrochemical feedstock are removed by converting them to hydrogen sulfide. Therefore, the reaction conditions and the catalyst formulations used for the second-stage hydrogenation operated at the small-sized naphtha steam crackers are different from those used for the above-mentioned second-stage hydrogenation of the chemically synthesized DHDCPD, DHMDCPD, and DHDMCPD. The second-stage hydrogenation catalyst for pygas is typically prepared by first combining the high surface area CoMo oxide catalyst with the high surface area NiMo oxide catalyst to form a CoMo/NiMo oxide dual catalyst system, followed by transforming the CoMo/NiMo oxide dual catalyst system to a sulfide state before hydrotreating in order to increase the catalytic hydrogenation activity of the dual catalyst system. Therefore, the catalytic hydrogenation activity of the dual catalyst system can be maintained for a long time if the fuel that is to be hydrotreated contains a small amount of sulfur containing species. This characteristic of the sulfided CoMo/NiMo dual catalyst system is different from that of the nickel or palladium metallic catalyst used for hydrogenation of the sulfur-free feedstock, such as the chemically synthesized DHDCPD to THDCPD. The hydrogenation catalyst for long-term use is required because the hydrogenation of pygas is typically carried out in a fixed bed reactor in continuous operation mode. However, because the hydrogenation of the high-density liquid hydrocarbons, which are chemically synthesized, is usually carried out in a batch reactor, the hydrogenation catalyst is not required to be used for a long-term period and is not required to resist sulfur poisoning.
A small amount of DCPD is always contained in the C5+ fractions (the term C5+ refers to the hydrocarbons containing five or more carbon atoms) obtained as a by-product from naphtha steam cracker for ethylene and propylene production. FIG. 1 shows a typical gas chromatograph of C5+ fraction of the pygas. FIG. 2 shows a typical gas chromatograph of C9+ fraction of the pygas. DCPD can be obtained by heat-dimerizing CPD, which is separated from the product stream in a distillation tower. FIG. 3 shows a typical gas chromatograph of C9+ fraction of the pygas after the first-stage hydrogenation. FIG. 4 shows a typical gas chromatograph of another C9+ fraction of the pygas after the first-stage hydrogenation. Referring to FIGS. 3 and 4, after the pygas is subjected to the first-stage hydrogenation at low temperature, DHDCPD and THDCPD, which are the hydrotreated products of DCPD, were usually found in the C10 fraction. However, the high purity DCPD and its hydrogenated products, namely DHDCPD and THDCPD used as a precursor of JP-10, have not been successfully separated from the other hydrocarbons in the pygas after the hydrogenation so far because there are residual sulfur impurity and a large number of C9-C10 aromatic hydrocarbons contained in the pygas, and the C9-C10 aromatic hydrocarbons have a similar or identical separation characteristics, such as boiling point, as that of DHDCPD and THDCPD. With the same reasons, the high purity DMCPD and its hydrogenated products, namely DHDMCPD and THDMCPD used as precursors of RJ-4 and RJ-4I, have not been successfully separated from the other hydrocarbons in the pygas after the hydrogenation so far. Likewise, the new high purity C11 hydrocarbons as precursors of high energy fuels have not been successfully separated from the other hydrocarbons in the pygas after the hydrogenation so far.
The pygas is not stable because a trace amount of highly reactive impurities, such as alkynes, diolefins, styrene, and methylstyrene, are present in the pygas. In order to stabilize the pygas, partially (the first stage) hydrogenation of C5-C8 fraction having a boiling point below 150° C. was carried out at a temperature of 50 to 110° C., a pressure of 400 to 600 psig H2, a liquid hourly space velocity of 1 to 4, an H2/liquid chargestock mole ratio of 1 to 2, and in the presence of a nickel or palladium catalyst, and alternatively partially hydrogenating the non-aromatic unsaturated moieties of the combination of the C5-C12 derivative and the C6-C8 fraction is performed at a temperature of 180 to 250° C., a pressure of 500 to 1000 psig H2, a liquid hourly space velocity of 1 to 2, an H2/liquid chargestock mole ratio of 8 to 12, and in the presence of a sulfided CoMo/NiMo single or dual catalyst system, and thereby the highly reactive components, such as alkynes, diolefins, and the like, in the pygas will be partially converted to alkenes. Furthermore, the C5 alkenes are separated from the alkenes for further use, and subsequently the C6-C8 fraction are separated from C5 alkenes. The second stage hydrogenation of C6-C8 fraction was carried out at a temperature of 260 to 350° C., a pressure of 300 to 1000 psig H2, a liquid hourly space velocity of 1 to 2, an H2/liquid chargestock mole ratio of 8 to 15, and in the presence of a sulfided CoMo/NiMo single or dual catalyst system, so that the aromatic alkenes are converted to the aromatic alkanes, and the sulfur-containing impurities are also removed from the products. Subsequently, C6-C8 aromatic hydrocarbons (such as benzene, toluene, and xylenes) were separated out by distillation or extraction, and were used as petrochemical feedstocks. Because a small amount of C9-C10 fraction contained in the pygas has a variety of constituents and has relatively high sulfur content, its stability is too low to be used. If the C9-C10 fraction was not hydrotreated or desulfurized, it was usually blended with low-quality gasoline, diesel oil, or kerosene so that its economical benefit is low. FIG. 5 is a block flow diagram of a sequence of steps for preparing C6-C8 aromatic hydrocarbons, C9+ aromatic hydrocarbons, crude fuel oil, and C9 resin at naphtha steam cracker. The C4− gas fraction is separated from the pygas by distillation. The pygas stream routed from steam cracker was proceeded to a depentanizer (V-11) to separate a C5 fraction from the other hydrocarbons, followed by proceeding to a deoctanizer (V-12) to separate an unhydrotreated C9+ bottoms from the pygas stream, and followed by proceeding to a denonanizer (V-13) to separate a overhead oil used as resin raw material from a bottoms as fuel. After the overheads obtained in the deoctanizer (V-12) was mixed with unhydrotreated C5 hydrocarbons, the diolefins and styrene contained in the mixture was selectively hydrogenated at the first-stage hydrogenation plant (R-11), followed by proceeding to a downstream depentanizer (V-21) to separate C5 olefins to be used as a feedstock for producing tert-amyl methyl ether. The C6-C8 and C9+ fractions were separated out from the C5 overheads of V-21 in the downstream deoctanizer (V-22). The C6-C8 fraction as V22 overheads was hydrotreated at the second-stage hydrogenation plant (R-21), followed by proceeding to a depentanizer (V-23) to recover the bottoms containing the C6-C8 aromatic hydrocarbons. Then, the bottoms containing the C6-C8 aromatic hydrocarbons were subjected to the separation followed by conversion to produce the petrochemical products. The C9+ fraction obtained from the bottoms of V-22 was blended with gasoline in a small proportion.
The unhydrotreated C9+ oil contained in the pygas stream can be used as the resin raw material, or can be hydrotreated and subsequently blended with gasoline. Either the first-stage hydrotreated C9+ fraction or the unhydrotreated C9+ fraction contains a large amount of aromatic hydrocarbons and unsaturated hydrocarbons that are detrimental to the quality of the fuels. However, the C9+ fraction is not suitable to be blended with gasoline according to the environmental regulation issued nowadays.
The cost for preparing the high energy fuels, such as JP-10, RJ-4, and RJ-4I, mainly depends on the cost of the chemical synthesis of the extra-high purity precursors of the C10 compounds, such as DCPD, DHDCPD, and THDCPD, or mainly depends on the cost of the chemical synthesis of the extra-high purity precursors of the C12 compounds, such as DMCPD, DHDMCPD, and THDMCPD. However, the cost of the chemical synthesis of the extra-high purity precursors of the C10 compounds or the C12 compounds is very high. Therefore, how to prepare the high energy fuels (such as JP-10, RJ-4, and RJ-4I) at cheap price becomes an important issue.